Fixed streaming tools (returning AsyncGenerator) failing schema generation with from __future__ import annotations and causing serialization errors during execution.
This change ensures string type hints are correctly resolved to identify streaming tools and consumes the async_generator result into a list before returning, making it serializable.
Added regression tests for both schema generation and tool execution.

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Streaming tools example in the documentation does not work #3955

2. Or, if no issue exists, describe the change:

Problem:

1. Schema Generation: When from __future__ import annotations is used, type hints are stored as strings. The FunctionTool schema generator failed to resolve these string annotations for AsyncGenerator return types, causing it to fail to identify the tool as streaming.
2. Serialization: FunctionTool.run_async returned the raw async_generator object. The ADK runner attempted to serialize this object for event logging, raising a TypeError because async_generator objects cannot be pickled.

Solution:

1. Schema Generation: Modified build_function_declaration in src/google/adk/tools/_automatic_function_calling_util.py to reuse the existing annotation_under_future dictionary (which contains resolved type hints). This correctly identifies AsyncGenerator types even when annotations are strings.
2. Serialization: Updated FunctionTool.run_async in src/google/adk/tools/function_tool.py to detect if the function result is an AsyncGenerator. If so, it consumes the generator into a list and returns the list of yielded items, ensuring the result is serializable.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.

• All unit tests pass locally.

• tests/unittests/tools/test_build_function_declaration.py : Verified schema generation for string-annotated AsyncGenerator.

• tests/unittests/tools/test_function_tool.py : Verified run_async consumes generator and returns list.

Manual End-to-End (E2E) Tests:

Verified locally with a reproduction script that successfully generated the schema and executed the tool without serialization errors.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.